Sinai Billiard

The simple Sinai billiard table is a flat square with a circular wall located at its center. It was introduced by Yakov G. Sinai as an example of a system in which all trajectories are ergodic and highly sensitive to the initial position and angle of direction of the ball. This model is sometimes called the Lorentz gas and it is usually seen as the reflection of a ray of light by an arrangement of perfectly reflecting circles in the plane.